Coherent rhythmic activity, either 10-15/sec. (type A) or faster (type B), is readily detected in somatosensory thalamus and cortex of acutely studied, lightly sedated and paralyzed cats. It is one manifestation of the functional interaction of central pathways converging onto somatosensory neurons at these levels. The identity of convergent pathways and of constituent individual neurons will be determined by conventional electrophysiological techniques, including tests of antidromic invasion and of monosynaptic activation.. Autocorrelation and cross-correlation functions will be computed on single unit discharges and somatosensory thalamic and cortical slow potentials, to quantify the periods and phase relations of rhythmic activity. The activity of members of different neuron populations (corticothalamic, pyramidal tract, intracortical, midbrain reticular, thalamic interneuronal) will be cross-correlated with the thalamic and cortical slow potential activity, to test hypotheses about the roles of these neurons in the generation of rhythmicity and in the central regulation of spontaneous activity of thalamocortical projection neurons. The central and peripheral receptive properties of corticothalamic neurons will be investigated. The interacortical laminar distribution of type A and type B rhythmic activity will be measured by computation of unnormalized cross-correlation functions between micropipette tip and surface potentials. Analysis of periodic neuron discharge patterns not correlated with population activity will be carried out with the objectives of identifying the site of generation of the periodicity, and of differentiating experimentally between alternative models by which information about steady stimulation at the periphery might be coded centrally.